A much misunderstood situation,
The top denture is kept in by several conditions, including lack of air movement under the plate, the presence of saliva inside the plate, and the seal around the denture base from the rest of your mouth allowing neither air to escape out of nor enter under the denture base. Along with the borders around the denture embedding themselves into the soft tissue between your palette and your cheeks and throat.  All the while the denture resting on your hard palette in the middle of the roof of your mouth.
While your lower denture basically sits on what lower jaw bone covered with tissue remains, and being bombarded on all sides by moving muscles and soft tissues, not to mention your tongue.  Compared to the top denture resting in a neutral relatively quiet area, the lower denture is in a constantly moving environment, trying to move or dislodge it, especially the tongue.
Having said this,  the lower denture can be made stable or as stable as possible only if the surrounding length and thickness of it’s borders are correctly shaped to follow the contour of this environment and be relieved to allow the tongue and all the moving muscles to move.  This is not easy to do, but it is very rewarding to have it done well, and as a patient you should be offered this to be done correctly.
Many lower dentures that are very loose and ill fitting are because they have not been maintained properly, relined or rebased as needed.
The lower denture is in many, many ways different than the top denture and should not be compared to it.  It should also be treated separately because of all the different challenges that need to be conquered to make it as well fitting as possible, and this is possible if done correctly and your practitioner extends to you the time to do it .
Again, seek the advice of an experienced practitioner whom you are comfortable with, and are able to communicate with easily with.
A very good question ,thank you,